Patent Document 1 discloses an electronic key system that locks and unlocks a vehicle door in response to signals from a portable device held by a user. The system includes the portable device held by a user and a control apparatus mounted on a vehicle. The portable device includes first receiving means, which receives a transmission request signal, and first transmitting means, which transmits a response signal in response to the transmission request signal. The control apparatus includes second transmitting means, which transmits the transmission request signal at predetermined time intervals, and second receiving means, which receives the response signal. When receiving the response signal from the portable device, the control apparatus outputs a signal for unlocking the vehicle door, and when not receiving the response signal, the control apparatus outputs a signal for locking the vehicle door after a predetermined time has elapsed from when the transmission request signal is transmitted.
In the above-mentioned system, a receivable range in which the portable device can receive the transmission request signal, that is, a range in which the portable device can transmit the response signal is determined by the power level (transmission output level) of the transmission request signal to be transmitted from the second transmitting means, and has an influence on the convenience of the system. That is, if the receivable range of the transmission request signal is too narrow, the vehicle door might not be unlocked even when the user approaches the vehicle to get on the vehicle. In contrast, if the receivable range of the transmission request signal is too wide, the vehicle door might be unlocked even when the user is not intending to get on the vehicle. Therefore, the receivable range of the transmission request signal, that is, the transmission output level needs to be maintained appropriate and constant.
The control apparatus operates in accordance with voltage supplied from a battery mounted on the vehicle. The voltage of the battery fluctuates depending on the condition of use and the ambient temperature of the battery. If the voltage supplied from the battery to the control apparatus fluctuates, the output level of the transmission request signal, that is, the receivable range of the transmission request signal fluctuates. Measures for maintaining the voltage supplied to the control apparatus include a measure in which a stabilized power supply circuit, which reduces the voltage of the battery, is provided between the battery and the control apparatus, and the control apparatus is operated by the output voltage of the power supply circuit. In addition, as another measure, Patent Document 2 discloses a measure in which an automatic power control circuit is used. The circuit includes a switching power supply that controls the gain of an amplifier, which amplifies the transmission request signal, in accordance with the transmission output level.
However, since the stabilized power supply circuit and the switching power supply are constantly operated, the battery voltage is reduced due to consumption of power by the stabilized power supply circuit and the switching power supply. Furthermore, the stabilized power supply circuit converts the battery voltage to a lower voltage. When using a dropper regulator for conversion of the voltage, energy loss such as heat generation occurs, and the battery voltage is not efficiently used. When the stabilized power supply circuit is configured by a switching regulator, the configuration of the circuit of the control apparatus is complicated, which enlarges the control apparatus. Since there is a restriction in the size of the control apparatus mounted on the vehicle, the enlarged control apparatus might not satisfy the restriction.    Patent Document 1: Japanese Laid-Open Patent Publication No. 5-106376    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-87069